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Two monitors from one USB-C port: what decides it

By Bulan Sarkar, founder of ElectronicsInfoline (since 2000)

Plug a dual-HDMI hub into one laptop and you get two screens; plug it into another and you get the same picture twice. Two things decide it: whether the port can send separate streams, and whether the lanes it gives to video carry both screens. Both come from VESA's figures.

You need MST support on the port (or Thunderbolt) for two different pictures, and enough bandwidth for both screens on the lanes the hub uses for video. Two 1440p60 screens need 11.26 Gbit/s: they fit on two lanes of DisplayPort 1.4 (12.96 Gbit/s), not on two lanes of DisplayPort 1.2 (8.64).

Two different pictures need MST

Multi-Stream Transport carries several independent display streams over one cable, to a hub or a daisy chain (VESA), and the streams share the link's bandwidth (VESA).

One DisplayPort link split into two streamsThe laptop sends two independent video streams over one DisplayPort link; the hub splits them to two monitors. Both streams share the link's bandwidth.Laptopone link, two streamsHubScreen AScreen B
Figure 1. With Multi-Stream Transport (MST) one DisplayPort link carries separate streams for each monitor, and they share its bandwidth (VESA). Without MST, a hub can only show the same picture on both.

Lanes decide the bandwidth

Alt Mode can put video on two of USB-C's four lanes, leaving two for USB 3 data (VESA), or on all four, with USB 2.0 on its own pins (VESA). A hub with USB 3 ports takes the first way, so it has half the video bandwidth.

USB-C DisplayPort Alt Mode: four lanes or twoTop: all four high-speed lanes carry DisplayPort; USB data runs at USB 2.0 on its own pins. Bottom: two lanes carry DisplayPort and two carry USB 3 data. Power runs alongside in both.Four lanes of videoDPDPDPDP+ USB 2.0+ powerMost video; USB data at USB 2.0 onlyTwo lanes of videoDPDPUSB 3USB 3+ USB 2.0+ powerHalf the video bandwidth; USB 3 data on the other two
Figure 2. VESA: Alt Mode can use all four high-speed lanes for video, with USB 2.0 data on separate pins, or two lanes for video and two for SuperSpeed USB data. A hub with USB 3 ports usually takes the second way, which halves the video bandwidth.

The sums for common pairs

Need per screen, from VESA reduced-blanking timing at 8-bit colour: 1080p60 3.2 Gbit/s, 1440p60 5.63, 4K60 12.54. Link capacity: two lanes of HBR2 8.64, two of HBR3 12.96, four of HBR3 25.92.

Video bandwidth needed against what the link carries2 × 1080p60: 6.4 Gbit/s; 2 × 1440p60: 11.26 Gbit/s; 2 × 4K60: 25.09 Gbit/s. HBR3 × 2 lanes: 12.96 Gbit/s.2 × 1080p606.4 Gbit/s2 × 1440p6011.26 Gbit/s2 × 4K6025.09 Gbit/sTotal42.75 Gbit/sHBR3 × 2 lanes: 12.96
Figure 3. Each bar is a pair of screens; the dashed line is two lanes of HBR3 (12.96 Gbit/s), the usual hub with USB 3 ports on a DisplayPort 1.4 laptop. Two 4K60 screens need 25.09 and only fit on all four HBR3 lanes (25.92), tightly.

VESA's own example agrees with the 1440p row: two lanes of HBR3 support two 2560 × 1440 screens at 60 Hz (VESA). Two 4K60 screens on four HBR3 lanes come out “with limits” in the checker: 25.09 of 25.92 Gbit/s.

Tips and tricks

  • Search the laptop’s spec sheet for “MST”, “multi-stream” or the number of external displays per port.
  • A hub listed as “dual 4K” may mean 4K at 30 Hz. Look for the refresh rate next to the resolution.
  • If one port cannot do it, two ports may: one monitor per USB-C port avoids sharing a link at all.
  • For a Thunderbolt 4 laptop, a Thunderbolt dock is the safe route: Intel promises two 4K60 displays.

What we would do

Check MST on the laptop's spec sheet, then run the pair through the hub and dock checker. For two 1080p or 1440p screens at 60 Hz, a hub with USB 3 ports is fine on a DisplayPort 1.4 laptop. For two 4K60 screens, use Thunderbolt 4 or 5, or one port per screen.

Questions people ask

Why do both my monitors show the same picture? Two different pictures over one USB-C video link need DisplayPort Multi-Stream Transport (MST), or Thunderbolt. Without it, a dual-output hub can only mirror. Check the computer maker’s spec sheet for MST on that port.

Can a USB-C hub run two 4K monitors at 60 Hz? On DisplayPort Alt Mode it takes all four lanes of HBR3: two 4K60 screens need 25.09 Gbit/s and four HBR3 lanes carry 25.92, which is tight. With two lanes (a hub with USB 3 ports) it does not fit. Thunderbolt 4 is promised two 4K60 displays by Intel.

Do two 1080p monitors work on any USB-C hub with video? By bandwidth, yes: two 1080p60 screens need 6.4 Gbit/s and even two lanes of HBR2 carry 8.64. The port still needs MST for two different pictures.

Sources

Each figure on this page is quoted from the source below or worked out from them. Sources checked , each quote read again against the source in a second pass.

  1. VESA, VESA® Brings DisplayPort™ to New USB Type-C Connector - VESA - Interface Standards for The Display Industry: By leveraging USB Type-C’s flexibility, the DisplayPort Alt Mode can choose to transmit on just one or two of the four available lanes, so that the other two lanes can be used for SuperSpeed USB data at the same time. vesa.org/featured-articles/vesa-brings-displayport-to-new-us…
  2. VESA, VESA® Brings DisplayPort™ to New USB Type-C Connector - VESA - Interface Standards for The Display Industry: When using all four lanes for DisplayPort Alt Mode, which could drive a monitor with up to 5K (5120 x 2880) resolution, USB 2.0 data can still be carried across the USB Type-C connection using separate pins dedicated for that function. vesa.org/featured-articles/vesa-brings-displayport-to-new-us…
  3. VESA, VESA® Introduces DisplayPort™ v1.2 - DisplayPort: DisplayPort v1.2 supports “multi-streaming”—the ability to transport multiple independent uncompressed display and audio streams over a single cable, supporting protected content and high performance applications such as 3D gaming. This enables the use of multiple monitors connected by cable in a daisy chain or hub configuration. displayport.org/pr/vesa-introduces-displayport-v1-2-the-most…
  4. VESA, VESA Rolls Out Early Certification Program for Video Source and Display Products Using HBR3 High-Bandwidth Link Rate - VESA - Interface Standards for The Display Industry: By using the DisplayPort Multi-Stream feature, this bandwidth can also be shared among multiple monitors connected to a dock. vesa.org/featured-articles/vesa-rolls-out-early-certificatio…
  5. VESA, Welcome VESA Workshop Tokyo, Japan 2024 (VESA PDF, page 10, 'DP Standard Version Number...'): • DPv1.1 = HBR (2.7 Gbps/lane) • DPv1.2 = HBR2 (5.4 Gbps/lane) • DPv1.4 = HBR3 (8.1 Gbps/lane) • DPv2.x = UHBR rates (10/13.5/20 Gbps/lane) vesa.org/wp-content/uploads/2024/10/VESA-Tokyo-Workshop_Octo…
  6. VESA, DP 1.3 Overview for VESA v1 (VESA/DisplayPort.org PDF, page 4, 'DP 1.3 Link Rate Increase'): 8.1 Gbps link rate, per lane x 0.8 to account for 8b/10b transport coding overhead x 4 maximum number of available lanes 25.92 Gbps total usable data transfer rate displayport.org/wp-content/uploads/2014/09/DP-1.3-Overview-f…
  7. VESA, Why DisplayPort - VESA - Interface Standards for The Display Industry: Up to 8.1 Gbps per lane–32.4 Gbps over 4 lanes Allowing for overhead — 25.92 Gbps over 4 lanes vesa.org/displayport-developer/why-displayport/
  8. VESA, VESA Display Summit USA 2016 (AMD/VESA presentation), timing table: 2 lane at HBR3 can support 2 x 2560x1440@60Hz using CVT Reduced Blank 1 vesa.org/wp-content/uploads/2016/06/Display-Summit-USA-2016-…
  9. VESA, VESA Display Summit USA 2016 (AMD/VESA presentation), timing table: 5120 x 2880 60 Hz IT No DP 1.3 VESA CVT RB 2 924.144 MHz vesa.org/wp-content/uploads/2016/06/Display-Summit-USA-2016-…
  10. Intel, What Is Thunderbolt™ 4 and How Is It Different from USB-C? – Intel: Up to two 4K 60 Hz displays or one 8K 60 Hz display on a single connection intel.com/content/www/us/en/architecture-and-technology/thun…

Author: Bulan Sarkar · Report an error

Numbers here are quoted from standards bodies and makers, or worked out from them. Nothing on this page was measured or tested by us.